xt+1 = 1.0 + yt - A xt2
yt+1 = B xt

parameters: A = 1.4, B = 0.3

Lyapunov exponents (log with base-e): 0.419118, -1.623091
Lyapunov Dimension:  1.258222

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sum of the Lyapunov exponents: -1.2039728043

Jacobian determinant: -1.2039728043

#----------------------------------------------

The Jacobian determinant of the Hénon map is −B.  Therefore, the sum of the Lyapunov exponents is theoretically equal to ln|B|.

Reference

A two-dimensional mapping with a strange attractor

Michel Hénon

Communications in Mathematical Physics, volume 50, 69–77 (1976)

DOI: 10.1007/BF01608556

Computing the Lyapunov spectrum of a dynamical system from an observed time series

Reggie Brown, Paul Bryant, and Henry Don Isaac Abarbanel

Physical Review A 43, 2787-2806 (1991)

DOI: 10.1103/PhysRevA.43.2787